The various mechanisms of X-ray radiation from relativistically charged particles in a crystal are analyzed from a common point of view, based on quantum electrodynamics in a medium. Parametric X-rays (PXR), diffraction radiation (DR) and coherent bremsstrahlung (CB) lead to different contributions to the amplitude of the radiation process but because of their interference they cannot be considered separately in the radiation intensity. The role of the dynamical diffraction effects and the coherent bremsstrahlung is considered to be dependent on the crystal parameters and particle energy. The conception of the high resolution parametric X-rays (HRPXR) is introduced and the universal radiation distribution, which can simplify the analysis of the results for this case is also considered. The possible applications of HRPXR are discussed.